Baclofen is a skeletal muscle relaxant and antispastic agent. Baclofen is a structural analog of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA), and may exert its effects by stimulation of the GABAB receptor subtype.
Lioresal® Intrathecal (baclofen injection) has been developed for chronic intrathecal infusion for the management of severe spasticity. Baclofen can be administered orally, but when injected directly into the intrathecal space of a patient, therapeutically effective intrathecal concentrations are achieved with resultant plasma concentrations 100 times less than those occurring with oral administration. Baclofen injections (Lioresal® Intrathecal, Medtronic, Inc.) are therefore commonly administered intrathecally to manage severe spasticity of spinal cord origin. Presently, baclofen is commercially available for intrathecal injection, as a 0.05 mg/mL solution, a 0.5 mg/mL solution or a 2 mg/mL solution having a pH of 5 to 7 in the following preservative-free formula (Lioresal® Intrathecal): baclofen (0.05 mg, 0.5 mg or 2 mg); sodium chloride (9 mg); water for injection q.s. 1 mL.
The 0.5 mg/mL and 2 mg/mL concentrations have proven to be very effective for delivery via an implantable infusion device, such as Medtronic, Inc.'s SynchroMed II® infusion device, for a number of patients suffering from severe spasticity. Higher concentration solutions are desired to reduce the frequency with which the reservoir of the implantable infusion device needs to be refilled.
However, since the solubility of baclofen in water is limited, aqueous solutions having higher concentrations of baclofen have not yet become commercially available. At higher concentrations, baclofen may not entirely dissolve in aqueous solution, or it may have an unacceptable tendency to precipitate out of solution during storage.
An upper limit on room temperature aqueous solubility of baclofen has been reported by some sources to be about 4 mg/mL, however such a concentration appears to require an equilibrium state in which particulate baclofen is present at some level.
Commercially and pharmaceutically acceptable injectable baclofen solutions for parenteral use or to be delivered via an infusion system desirably will be essentially free from particles that can be observed on visual inspection and the baclofen will stay in solution without precipitating prior to and during administration to a patient.
Recent reports suggest that higher concentrations of baclofen without significant precipitation of particles can be achieved when baclofen is dissolved in hot water and cooled, or baclofen is dissolved in highly acidic or basic solutions and back-titrated towards neutral pH. However, it is desired for injectable baclofen solutions to have no detectable (observable on visual inspection) amount of particulates.
Injectable baclofen solutions will also desirably have a limited amount of degradation products. The main decomposition product of baclofen is 4-(4-chlorophenyl)-2-pyrrolidine (4-CCP). For example, one of the currently available injectable baclofen solution described above (Lioresal® Intrathecal) generally contains about 0.6% of 4-CPP (relative to the starting concentration of baclofen). The rate conversion of baclofen to 4-CPP is known to increase at increased temperatures. Injectable baclofen solutions are desirably sterilized. However, if heat sterilization is used in manufacturing, this may result in an undesirably increased amount of degradation products.
It would therefore be desirable to provide new baclofen formulations and sterilization processes of injectable baclofen formulations that limit the production of 4-CPP.